Magnetoresistive spin valve sensor with stepped layers

ABSTRACT

A magnetoresistive spin valve sensor is described. Such a sensor is also known as a GMR sensor or giant magnetoresistive sensor. The layers (24, 26, 28) of the sensor are mounted on a substrate (20) having steps or terraces on one of its face. The steps or terraces on the substrate&#39;s surface cooperate with one or more of the ferromagnetic layers (24, 28) of the sensor to determine the layers&#39; magnetic properties. Specifically, the thickness of one or more of the sensor&#39;s layers can be set above or below a critical thickness which determines whether the easy direction of uniaxial magnetization of a layer of that particular material is fixed or &#34;pinned&#34;. If pinned, the layer has a high coercive field. Thus, the new device avoids a biasing layer to pin any of the magnetic layers. Preferably the easy axes of the first two ferromagnetic layers (24, 28) are set at 90° to one another in the zero applied field condition by appropriate choice of layer thickness. A method for manufacturing and several fields of use of the sensor are also disclosed.

The present invention relates generally to magnetoresistive sensors, inparticular to magnetoresistive sensors based on the so-called"spin-valve" or "giant magnetoresistive (GMR)" effect. The inventionfurther relates to storage systems incorporating such sensors forreading stored information. The magnetoresistive sensor of the presentinvention is also applicable to any localized measurement task orproblem where a magnetic field is to be detected in a restricted volumeor area.

BACKGROUND OF THE INVENTION

The prior art discloses a magnetic read transducer referred to as amagnetoresistive (MR) sensor or head which has been shown to be capableof reading data from a magnetic surface at great linear densities. An MRsensor detects magnetic field signals through the resistance changes ofa read element fabricated of a magnetic material as a function of thestrength and direction of magnetic flux being sensed by the readelement. These prior art MR sensors operate on the basis of theanisotropic magnetoresistive (AMR) effect in which a component of theread element resistance varies as the square of the cosine of the anglebetween the magnetization and the direction of sense current flowthrough the element. A more detailed description of the AMR effect canbe found in "Memory, Storage, and Related Applications", D. A. Thompsonet al., IEEE Trans. Mag. MAG-11, p. 1039 (1975).

More recently, a different, more pronounced magnetoresistive effect hasbeen described in which the change in resistance of a layered magneticsensor is attributed to the spin-dependent transmission of theconduction electrons between the magnetic layers through a non-magneticlayer and the accompanying spin-dependent scattering of electrons at thelayer interfaces and within the ferromagnetic layers. Thismagnetoresistive effect is variously referred to as the "giantmagnetoresistive" (GMR) or "spin valve" effect. Such a magnetoresistivesensor fabricated of the appropriate materials provides improvedsensitivity and greater change in resistance than observed in sensorsutilizing the AMR effect. In this type of MR sensor, the in-planeresistance between a pair of ferromagnetic layers separated by anon-magnetic layer varies as the cosine (cos) of the angle between themagnetization in the two layers.

U.S. Pat. No. 4,949,039 to Grunberg describes a layered magneticstructure which yields enhanced MR effects caused by antiparallelalignment of the magnetizations in the magnetic layers. Grunbergdescribes the use of antiferromagnetic-type exchange coupling to obtainthe antiparallel alignment in which adjacent layers of ferromagneticmaterials are separated by a thin interlayer of Cr or Y.

U.S. Pat. No. 5,206,590 to Dieny et al. discloses an MR sensor in whichthe resistance between two uncoupled ferromagnetic layers is observed tovary as the cosine of the angle between the magnetizations of the twolayers. This mechanism produces a magnetoresistance that is based on thespin valve effect and, for selected combinations of materials, isgreater in magnitude than the AMR.

The U.S. Pat. No. 5,159,513 to Dieny et al. discloses an MR sensor basedon the above-described effect which includes two thin film layers offerromagnetic material separated by a thin film layer of a non-magneticmetallic material wherein at least one of the ferromagnetic layers is ofcobalt or a cobalt alloy. The magnetization of the one ferromagneticlayer is maintained perpendicular to the magnetization of the otherferromagnetic layer at zero externally applied magnetic field byexchange coupling to an antiferromagnetic layer (element 18 in FIG. 2 ofU.S. Pat. No. 5,159 513)

Published European Pat. Application EP-A-0 585 009 discloses a spinvalve effect sensor in which an antiferromagnetic layer and an adjacentmagnetically soft layer co-operate to fix or pin the magnetization of aferromagnetic layer. The magnetically soft layer enhances the exchangecoupling provided by the antiferromagnetic layer.

The spin valve structures described in the above-cited U.S. patents andEuropean patent application require that the direction of magnetizationin one of the two ferromagnetic layers be fixed or "pinned" in aselected orientation such that under non-signal conditions the directionof magnetization in the other ferromagnetic layer, the "free" layer, isoriented either perpendicular to (i.e. at 90° to) or antiparallel to(i.e. at 180° to) the direction of magnetization of the pinned layer.When an external magnetic signal is applied to the sensor, the directionof magnetization in the non-fixed or "free" layer rotates with respectto the direction of magnetization in the pinned layer. The output of thesensor depends on the amount of this rotation. In order to maintain themagnetization orientation in the pinned layer, a means for fixing thedirection of the magnetization is required. For example, as described inthe above-cited prior art documents, an additional layer ofantiferromagnetic material can be formed adjacent to the pinnedferromagnetic layer to provide an exchange coupled bias field and thuspinning. Alternatively, an adjacent magnetically hard layer can beutilized to provide hard bias for the pinned layer.

Another alternative to provide a ferromagnetic layer with a fixed orpinned orientation is described in U.S. Pat. No. 5,301,079 granted toCain et al. A magnetoresistive read sensor based on the spin valveeffect is disclosed in which a sense current flowing in the sensorelement generates a bias field which sets the direction of magnetizationin each ferromagnetic layer at an equal, but opposite, angle θ withrespect to the magnetic easy axis, thus providing an angular separationof 20 in the absence of an applied magnetic signal. Application to thissensor of the magnetic signal to be sensed results in a furtherincremental rotation of the direction of magnetization of eachferromagnetic layer, the directions of these two incremental rotationsbeing opposite.

With regard to the above prior art, it is a principal object of thepresent invention to provide an MR sensor based on the spin valve effectin which neither an antiferromagnetic layer nor hard biasing, nor biascurrent generating circuitry is required for fixing the magnetizationorientation in one or more of the ferromagnetic layers.

SUMMARY OF THE INVENTION

The above-mentioned and other objects and advantages are attained inaccordance with the principles of the present invention as set forth inthe appended claims.

In accordance with the present invention, the topology of the substrateis such that the substrate's surface consists of regions forming aseries of steps or terraces with parallel upper surfaces. These stepsmay "run" in only one or in more than one direction across thesubstrate's surface. They may be such that, viewed as a whole, thesurface of the substrate slopes in only one, or in more than onedirection in relation to the "level" defined by the top of a particularone of the regions. A "miscut" crystalline substrate, which has been cutto have it's upper surface non-parallel to the angularly nearestcrystallographic plane, may be used as such a substrate.

A spin valve sensor also in accordance with the present invention cancomprise a substrate, the face of said substrate on which the firstferromagnetic layer of the said sensor is mounted having a plurality ofsteps, whose mean length preferably lies between 10 and 1000 Å (1-100nm), the thickness of one of the sensor's ferromagnetic layers beingsuch that layer's magnetization direction is pinned. A "pinned" layer isone whose anisotropy favors one direction and which has a high enoughcoercive field that this direction is not changed under the influence ofmagnetic fields encountered in use. Preferably, the mean step height isbetween 1.5 and 30 Å (0.15-3.0 nm).

Also in accordance with the present invention, a spin valve sensor cancomprise a substrate, the face of said substrate on which the firstferromagnetic layer of the said sensor is mounted having a plurality ofsteps, the mean ratio of the step length to the step height of saidsteps being between 5 and 570, the thickness of one of the sensor'sferromagnetic layers being such that that layer's magnetizationdirection is pinned.

Further in accordance with the present invention, a spin valve sensorcan comprise a crystalline substrate, wherein the face of the substrateon which the layers of the sensor are mounted is not coplanar with anyof the crystallographic planes of the substrate, the thickness of one ofthe sensor's ferromagnetic layers being such that that layer'smagnetization direction is pinned. In such a sensor, the face of thecrystalline substrate on which the layers of the sensor are mountedadvantageously lies in a plane, the normal to which plane makes an angle(α) of at least 0.1°, preferably at least 0.5°, to the nearest crystalaxis of said crystalline substrate.

The nearest crystal axis referred to above is the axis of the nearestmain crystal plane. This plane is the particular one of thecrystallographic planes whose normal is closest to the direction of thenormal to the plane in which the surface of the crystal lies. As thenormals of the main crystal planes, i.e. those denoted by conventionwith small Miller indices, e.g. smaller than 3 (<100> directions, 1-10!,110!, 211!, 221!, etc.), make angles of at least about 15° to eachother, a skilled person can readily determine the nearest crystal planeto a given or desired normal to the face of the substrate.

Substrates whose surfaces have been deliberately cut at a small angle tothe direction of the nearest atomic plane of the crystal may be used toobtain a substrate falling within the terms of the present invention.Alternatively, the plurality of steps on the surface of the substratecan be achieved by polishing, epitaxial growth of a buffer layer with aslightly mismatching crystalline structure onto a regular cut substrate,sputtering, and/or suitable doping. These latter techniques areparticularly advantageous for the production of substrates with curvedsurfaces, i.e. non-planar surfaces, and/or for substrates with repeatingpatterns of steps. Similarly, the layers of the sensor may be producedby sputtering, epitaxial growth or equivalent standard techniques. Thesubstrate may consist of an insulating or a semi-conducting material,and is preferably of silicon.

The sensor of the present invention relies on the properties of layerswhich are mounted on substrates whose surfaces are stepped or terraced,or have been cut at a small angle to the direction of the nearestcrystallographic plane. These properties have been determined by theinventors of the present application and are as follows: The easydirection of uniaxial magnetization (or "easy axis") and the coercivefield for a ferromagnetic layer on a terraced substrate depend on thethickness of the ferromagnetic layer. In particular, as the thickness ofthe ferromagnetic layer, being considered, changes, a critical thicknesswill be reached where, over a narrow thickness range, the easy directionof uniaxial magnetization will turn through 90° relative to itsorientation for thicknesses prior to reaching this critical thickness.Thus the easy direction of uniaxial magnetization of a ferromagneticlayer can be set by ensuring that it has a particular thickness. It hasalso been found that the coercive field of a ferromagnetic layer changesgreatly when the thickness of the layer passes a critical value. Thus itis possible to set the coercivity of a ferromagnetic layer by ensuringthat it has a particular thickness. For a given ferromagnetic material,the change by 90° of the easy direction of magnetization and the largechange in coercivity occur at well defined critical values offerromagnetic film thickness. The critical thickness value for thechange in the direction of the easy axis of magnetization may not be thesame as the critical thickness value for the jump in coercivity.

As an example of the above, a Cobalt layer mounted on a miscutcrystalline substrate was found to have a critical thickness in theregion of 45 Å (4.5 nm), above which thickness the easy direction ofuniaxial magnetization was perpendicular to that shown by layers oflesser thickness than this critical value. Also, the coercivity of aCobalt layer with thickness greater than 45 Å (4.5 nm) mounted on amiscut substrate is found to be far higher than that of a Cobalt layerof lesser thickness.

The property of high coercivity of a ferromagnetic layer of a particularthickness on a miscut substrate amounts to a fixing or pinning of theeasy direction of uniaxial magnetization of the layer. This isequivalent to the pinning discussed earlier in connection withmagnetoresistive sensors, however without there being any additionalexternal layer required to bring this about.

The high coercivity and the change in orientation of the easy directionof uniaxial magnetization through 90° can be advantageously employed byconstructing two ferromagnetic layers, not necessarily made of the samematerial, separated by a non-magnetic spacer layer on 'the samesubstrate, one ferromagnetic layer having a thickness such that it has ahigh coercivity and a 90° rotation of its easy direction of uniaxialmagnetization takes place, the other layer having a thickness such thatit has a low coercivity and this rotation does not take place. In otherwords, the thickness of one of the layers is above the criticalthicknesses for pinning and for the 90° rotation of the easy axis, andthe thickness of the other layer is below these critical thicknesses.Such an arrangement provides a spin-valve sensor. It is not howevernecessary for both layers to be made of materials whose magneticproperties depend on thickness. The sensor can consist of one layerwhose properties are thickness dependent, and one layer of a differentmaterial which constitutes either the free or pinned layer independentof its thickness.

The invention is not restricted to ferromagnetic layers whose coercivityincrease and change in easy axis occur above a critical or thresholdthickness. A spin-valve sensor can be made from materials where thesechanges occur for thickness values below a critical thickness. What isimportant is the possibility of setting the value of coercivity, and inpreferred embodiments also the easy direction of uniaxial magnetization,by setting the layers' thicknesses. In particular this leads, throughappropriate combinations of layer thickness, to fixing of the directionof magnetization of one of the layers whilst leaving the direction ofmagnetization of the other layer free to change. A spin-valve sensoraccording to the invention preferably also has the directions of itseasy axes set at an angle of 90° by appropriate choices of layerthickness in order to achieve optimum resistance change of the layersfor a given externally applied magnetic field to be sensed.

It is an advantage of the present invention that the spin-valve sensorin accordance with the invention can have two ferromagnetic layersseparated by a non-magnetic spacer layer, the pinned ferromagnetic layerneeding no additional pinning or biassing layer to fix or "pin" itsdirection of magnetization. Therefore the sensor according to theinvention shows enhanced sensitivity without requiring the fabricationof the extra pinning layer required by prior art spin valve sensors.This results in a device which is smaller and easier to fabricate thanknown spin valve sensors. The pinned layer may be the ferromagneticlayer closest to the face of the substrate on which the layers lie, inwhich case the second ferromagnetic layer, i.e. the further of the twolayers from the substrate, is the free layer. This arrangement mayhowever be reversed. In a further variant of the invention, a bufferlayer may be introduced between the substrate and the first magneticlayer, as to facilitate the deposition or as a protective layer.

The critical thickness values for layers to show the effects of theinvention depend on the materials involved and on the topology of thesubstrate. Therefore the invention is not restricted to ferromagneticpinned layers with the exact thicknesses of the examples in thisdescription, since these thickness values are only appropriate to oneparticular film substance and one particular topology. However, a methodis given enabling a skilled person to determine the critical thicknesseswhich mark onset and decay of the pinning effect and the 90° rotation ofthe easy axis for any combination of ferromagnetic material andsubstrate topology. Ferromagnets such as Co,Ni,Fe, or alloys of thesematerials with each other, e.g. permalloy, may be used for theferromagnetic layers. These materials may further be alloyed with othernon-magnetic materials. As already mentioned above, the pinned and freeferromagnetic layers do not have to both consist of the same material.

These and other novel features believed characteristic of the inventionare set forth in the appended claims. The invention itself however, aswell a preferred mode of use, and further objects and advantagesthereof, may be better understood by reference to the following detaileddescription of illustrative embodiments when read in conjunction withthe accompanying drawings.

DESCRIPTION OF THE DRAWINGS

The invention is described in detail below with reference to thefollowing drawings:

FIG. 1 shows the basic arrangement of a spin-valve sensor as is knownfrom the prior art.

FIG. 2 shows the basic arrangement of the spin-valve sensor according tothe present invention.

FIGS. 3-4 show substrates in accordance with the invention in threedimensional view.

FIGS. 5-12 show substrates in accordance with the invention incross-section.

FIG. 13 shows the coercive field of a ferromagnetic layer, illustratingthe pinning strength of the layer.

FIGS. 14 A-C show hysteresis loops of a free layer and of aferromagnetic layer which has been pinned in accordance with theinvention; and

FIGS. 15 A-B show magnetization images of the free and pinned layers ofa spin-valve sensor.

For consistency of presentation, FIGS. 3-6,8,11 and 12 have been drawnwith the substrate's base and the horizontal of the figure coincidingwith the level of the steps in the substrate's surface. However, theinvention also encompasses substrates whose bases are parallel to theplane in which the working surface of the crystal has been cut, i.e.substrates whose steps are tilted when viewed in cross-section with thehorizontal of the figure coinciding with the substrate's base.

MODE(S) FOR CARRYING OUT THE INVENTION

The GMR or `spin-valve effect` is based on the fact that themagnetoresistance of a pair of ferromagnetic layers separated by anonmagnetic spacer layer changes in dependence on the relativeorientation of the magnetization vectors in the ferromagnetic layers.The resistance is found to take an extreme value (i.e. maximum orminimum) when the magnetic layers have antiparallel magnetization, andthe opposite extreme value when the magnetizations of the layers arealigned parallel. This dependence of the resistance on the magnetizationconfiguration is caused by the inequality of the scattering rates ofspin-up and spin-down conduction electrons, be it at the interfacesbetween the layers or in the bulk.

An optimized sensor based on the GMR effect has its ferromagnetic layersin a single domain state, where the magnetization of one layer (the"pinned" layer) is fixed along a particular direction and themagnetization of the second layer (the "free" layer) can follow easilythe magnetic field to be sensed. If used to detect the state ofmagnetization of a recorded data bit, the free layer changes from moreparallel to more antiparallel alignment relative to the pinned layerwhen sensing successively two oppositely oriented bits. For optimizedlinear operation of the device with applied field the magnetization ofthe two layers should be at an angle of 90° to one another as long as nomagnetic field to be sensed is present. In the devices of the prior artthis is achieved by fixing the magnetization of the pinned layer alongits hard magnetization direction typically either by permanent magnetsor by exchange coupling to an antiferromagnetic layer, e.g. FeMn.

FIG. 1 shows an exploded view of such a prior art spin-valve sensor asknown for example from the U.S. Pat. No 5,159,523. However, the layersdiscussed in, and illustrated in FIG. 2 of U.S. Pat. No. 5,159,523 havebeen constructed in the reverse order to those shown in prior art FIG. 1of the present application.

Considering further FIG. 1 of the present application, reference 10indicates a substrate. Layer 14 mounted on the substrate is aferromagnetic layer, whose direction of magnetization is indicated bythe arrow on its surface. The direction of magnetization of layer 14 isfixed or "pinned" by the provision of an additional layer 12 consistingeither of an antiferromagnetic material or of a hard ferromagneticmaterial.

Layer 16 is a non-magnetic spacer layer. Layer 18 is a furtherferromagnetic layer, whose magnetization direction in the absence of amagnetic field to be measured is indicated by the horizontal arrow onits surface. Layer 18 is the free layer. The upwardly and downwardlyinclined arrows on layer 18 indicate directions in which themagnetization of the layer may lie under the influence of variousexternally applied magnetic fields to be measured. Such changes in thedirection of magnetization of layer lead, via the spin-valve effect, tochanges in the resistance of the group of layers 14,16, 18. Thisresistance is conventionally measured in the lengthwise direction of thelayers, i.e. between the left and right edges of the group of layers inthe orientation shown in FIG. 1.

When used to detect the state of magnetization of data points on amagnetic recording medium, the spin-valve sensor of FIG. 1 is positionedsuch that at least layer 18 is subjected to the magnetic fieldoriginating from the data point. The usual direction of movement of themagnetic recording medium is illustrated by the arrow in the foregroundof FIG. 1.

FIG. 2 shows an exploded view of a sensor according to the invention.The surface of the substrate 20 is terraced and will be described ingreater details below. The thickness of one of the ferromagnetic layers,24 or 28, is greater than the critical thickness necessary for the easydirection of uniaxial magnetization to be pinned. The otherferromagnetic layer has a thickness which is less than this criticalthickness, whereby its magnetisation direction can change when there isan externally applied magnetic field. This latter layer thereforeconstitutes the "free" layer. The thickness of one of the ferromagneticlayers, 24 or 28, should preferably be greater than the criticalthickness for its easy direction of uniaxial magnetization to be rotatedthrough 90°, and the other layer should preferably have a thicknesswhich is less than this critical thickness. Therefore the easy axes ofthe two ferromagnetic layers make an angle of 90° to each other whenthere is no externally applied magnetic field. This optimizes the outputsignal from the sensor, but other easy axis orientations areconceivable, e.g. a signal would still be generated if the axes wereparallel when there is no externally applied magnetic field to besensed. Alternatively, one of the layers may be made of a material whosemagnetic properties are not thickness dependent.

The principal difference of the arrangement according to the inventionand FIG. 2 from the arrangement of prior art FIG. 1 is that thearrangement of the invention does not need a pinning layer 12 to ensurepinning of the direction of magnetization of one of the ferromagneticlayers, 24 or 28. It is the surface structure of substrate 20 incombination with the thicknesses of the ferromagnetic layers whichensures pinning of the magnetization direction of one of the layers,whilst leaving the other layer as the free layer.

A method of determining the critical thicknesses for layers of variousferromagnetic materials is as follows: A number of substrates cut at aparticular angle α should be made, as this is the fastest way to producea terraced surface. Progressively thicker layers of ferromagneticmaterial should be deposited by any conventional method on differentones of these substrates, and each substrate/layer configuration testedfor direction of uniaxial magnetization and coercivity. At someparticular value(s) of layer thickness, the easy axis and coercivitywill change greatly between one configuration and the next one tested.This detects the critical thicknesses at which the easy direction ofuniaxial magnetization rotates through 90° and at which the coercivityrises (or falls) sharply.

If more precise thickness values for these transitions are desired, theexperiment can be repeated with another set of substrates, onto whichferromagnetic layers have been formed with thicknesses slightly greaterthan and slightly less than the critical thicknesses determined from thefirst "coarse" batch of substrates.

The above method can be repeated for other angles α. It is to beexpected that the critical thicknesses or the angles α of cut, or thedimensions of the steps respectively, of the substrate's surface whichlead to effective spin-valve sensors may differ from the numericalvalues given in this application, as different ferromagnetic materialsshould not be expected to behave identically to the Cobalt example citedin this application. The general applicability of the techniques of thepresent invention is such that the skilled person would expect toinvestigate the suitability of various materials and arrangements beforesettling on one particular combination of layer thicknesses, angle ofsubstrate surface cut and layer sequence to form a sensor for aparticular use. All such investigations form part of the scope andapplicability of the present invention, as this invention opens up awide field of possible sensor arrangements, simplified with respect toprior art sensors.

The largest angle of miscut of substrate for which the effects of theinvention have been checked is 6.0°. See example IV at the end of thisdescription. However, it is to be expected that the effects of theinvention will be observed up to miscuts of at east approx. 10° based onthe consistency of the effects observed in the range up to and including6.0°.

FIGS. 3-12 are schematic representations of substrates which lead to theeffects of the invention and which can be used as the substrate in thearrangement of FIG. 2. FIGS. 3-12 are enlargements which are not drawnto scale in all dimensions. The sequence of pinned ferromagnetic,non-magnetic and free ferromagnetic layers of a spin-valve sensor is tobe mounted onto these substrates. In FIG. 3, the regions of atoms on thesubstrate's surface take the form of steps or terraces. The uppersurface of each step comprises one of the layers of atoms of thecrystalline material of which the substrate is made. A crystallinesubstrate has been illustrated, which has the upper surfaces of its foursteps parallel to one another. The steps' upper surfaces consist ofregions of parallel planes of the crystal's atoms. The arrow labelled Sindicates the direction of the normal to the upper surface of a step.The arrow labelled P indicates the direction of the normal to the planein which the crystal's surface has been cut. The angle between thedirections indicated by arrows S and P is the angle of miscut of thesurface of the substrate or generally the inclination or slope angle ofthe terraced surface, α. An angle a of between 0.1° and 10° is believedto result in pinning along the required direction of an overlyingferromagnetic layer of a particular range of thicknesses, and thus tolead to the effects of the present invention. As explained in example IIat the end of this description, for a test sensor the range ofthicknesses 45-900 Å (4.5-90 nm) was found to be suitable for the pinnedlayer when this consisted of Co and the copper substrate had ainclination angle α of 1.6°. A further sensor was constructed with amiscut angle α of 1.9°, see example III. A slightly lower minimum pinnedlayer thickness appeared to work with a substrate miscut at 1.9°.

The dimension L indicated on the figures denotes the length of anindividual step. Likewise, the dimension H indicates the height of anindividual step. The dimension W illustrates only the width of theparticular section of substrate illustrated, and can generally beinterpreted as showing the width direction. Although the stepsillustrated in FIG. 3 have been shown to be approximately equal inlength, it is believed that the effects of the invention are achieved byany group of steps whose mean length lies within the range of 10-1000 Å(1-100 nm). Measurement of the sensor's resistance can either be made inany direction parallel to the plane of the layers, which is preferred,or in a direction perpendicular to the plane of the layers, which maygive an improved signal but be harder to implement.

FIG. 4 illustrates a substrate where the steps "run" in more than onedirection across the surface. This results from a miscut of the crystalsuch that the substrate's surface lies in a plane whose line ofintersection with the upper surface of the steps does not run parallelto any of the step edges shown in FIGS. 3 or 4. In this case, thesection of substrate surface shown in FIG. 4 will be repeated in the"into the paper" direction.

The various regions shown in FIGS. 3-12 consist of planes of atoms whichdiffer from one another in height by one or more atomic thicknesses or"monolayers". The substrates shown in FIGS. 3-12 induce pinning onoverlying ferromagnetic layers of appropriate thickness, provided thatthe mean length L of the steps on the whole surface is between 10 and1000 Å A (1 and 100 nm respectively), or that the mean ratio of the steplength to step height is between 5 and 570. The steps may be of equallength, but do not have to be. Typical step heights H are in the rangeof 1.5-30 Å (0.15-3.0 nm). The angle of miscut of the crystal which willgive rise to the effects of the invention is believed to extend between0.10 and 100. The angle is however preferably at least 0.5°. For asubstrate miscut at 0.5° the mean step length is approximately 200 Å (20nm) and the mean ratio of step length to step height is approximately110. For a substrate miscut at 6.0° the mean step length isapproximately 17 Å (1.7 nm), and the mean ratio of step length to stepheight is approximately 10.

In order to illustrate further topographical arrangements of substratesurfaces which may be employed to achieve the effects underlying theinvention, reference is made to FIGS. 5-12. FIGS. 5-12 showcross-sections of substrates. Each step height shown represents one ormore layers of atoms. The steps are shown as being "sharp" in thisillustration: in reality they are rounded or less regular due to thediscrete nature of the atoms of which they are made. The aspects of thesubstrates of FIGS. 5-12 which are of particular relevance to theinvention are:

FIG. 5: Steps of equal length.

FIG. 6: Steps of unequal length.

FIG. 7: Slopes in two opposed directions away from a central peak threedimensional view.

FIG. 8: Steps of unequal heights and unequal lengths.

FIGS. 9 and 12: No net slope, but a repeating pattern of steps.

FIG. 10 A variable gradient miscut, i.e. no single plane of miscut but acurved upper surface.

FIG. 11: Repeating pattern of unequal step length.

The geometry of each of FIGS. 5-12 may be repeated in the substratesurface direction not shown in the figure, i.e. in the "into the page"direction. Alternatively, the surface profile of the substrate in the"into the page" direction may take the form of that shown in any otherof FIGS. 5-12, in so far as substrate manufacturing techniques permitand provided that the symmetry of the arrangement remains at mosttwo-fold, particularly not four-fold, such that one preferredmagnetization direction can be obtained and fixed. Substrates accordingto the invention may fall within the terms of one or more of theappended independent claims, e.g. the substrate of FIG. 10 does not havea single plane as defined in claim 4 because the surface's substrate iscurved, but may be constructed with steps according to either of theappended claims.

The layers of the sensor may be constructed in the opposite order tothat shown in the generalized spin-valve sensor of FIG. 2, i.e. thepinned layer may be furthest from the substrate with the free layerlying between the substrate and the pinned layer. The pinned layerremains pinned in this configuration provided that its thickness iswithin the range necessary for pinning although the free andnon-magnetic layers lie between it and the substrate. Furthermore, thefree layer remains free and is not subject to pinning provided that itsthickness is not within the range necessary for pinning. The free layerin example II at the end of this description must have its thicknessbelow the critical thickness which marks the onset of pinning.

For similar reasons, a sensor may be constructed consisting, in order,of a substrate/pinned layer/non-magnetic layer/free layer/spacer/pinnedlayer/non-magnetic layer/free layer . . . etc., provided again that thethicknesses of the free and pinned layers used lie respectively belowand within the ranges necessary for pinning to occur, and that anon-magnetic spacer layer is provided between any free layer andadjacent pinned layer. Likewise the order substrate/freelayer/non-magnetic layer/pinned layer /spacer/free layer/non-magneticlayer/pinned layer/spacer etc. could be used, or substrate/pinnedlayer/non-magnetic layer/free layer/spacer layer/free layer/non-magneticlayer/pinned layer etc. Additional buffer layers may be introducedwhenever appropriate.

In particular, it is regarded as being well within the scope of theinvention for the substrate to be covered by an additional buffer layerbefore any of the ferromagnetic or nonmagnetic layers 14-18 as shown inFIG. 2 are deposited. Such a buffer layer becomes for example importantwhen a mismatch in the crystal structure of the buffer layer and the(original) substrate is exploited to fabricate a surface with steps--amethod which is described in the MRS Bulletin, June 1991, pp. 30--33 byC. P. Flynn. Additional shielding layers may also be provided by similartechniques.

The sensor of the present invention is particularly suitable to form themagnetic transducer in a magnetic storage system such as a hard diskdrive or similar device. The transducer reads the state of magnetizationof the magnetic recording medium, which moves close to the transducer ina plane perpendicular to, or at more than 80° to, the plane in which thelayers of the sensor lie.

A sensor according to the present invention may be used to detect thepresence of, or to measure, a magnetic field. In particular, the sensormay be constructed in a variety of sizes and can be expected to beespecially suited to detect the presence of, or for the measurement of,magnetic fields of 0.001-100 Oe (0.00008-8.0 kA/m). As the sensor can bemade very small, it is also particularly suited to the measurement anddetection of magnetic fields which are of limited spatial extent, orwhich exist in regions to which there is only limited access. Fields ofuse can thus be found in earth exploration like mining an oil drilling,or to measure small magnetic fields produced by the human body.

EXAMPLE I

A test structure in accordance with the invention consists of two Colayers separated by a nonmagnetic Cu spacer layer. A very small miscutof the nominally (100) oriented Cu substrate induces a large uniaxialmagnetic anisotropy in the Co film.

Considering test structures of progressively increasing thickness, at acertain thickness d_(c) the Co film abruptly switches its easymagnetization direction by 90° to a direction in which the orientationremains, even under the application of external magnetic fields of themagnitude to be detected by a spin valve sensor in use. The Co film hasthus been pinned and has shown a rotation of 90° of its easy axis. Forfilms with a thickness d>d_(c) the coercive field is enhanced by morethan a factor of two as compared to the thinner films. Therefore asensor according to the present invention can have as a pinned layer auniaxial Co film with d>d_(c), a Cu spacer layer thick enough to showonly very small coupling, and a free sensing layer.

In use as a detector of the state of magnetization of a data point on amagnetic storage medium, the free Co layer switches in the stray fieldof the written bits and hence induces the GMR effect by changing frommore parallel to more antiparallel alignment with the pinned Co layer.

In the above described test structure the sensing layer is a thin Colayer, but soft magnetic layers such as permalloy layers might bepreferable. The miscut of the substrate, i.e. the cutting of the uppersurface of the substrate such that that surface lies in a plane whichdoes not coincide with a unique plane of the crystal's atoms, gives riseto the aforementioned "regions" on the crystal's surface.

EXAMPLE II

Films of various thicknesses were grown on top of Cu<100> singlecrystals, miscut by 1.6° essentially along the in-plane 110! direction.This provoked a large uniaxial surface anisotropy, preferring the 1-10!direction over the 110! direction (which are equivalent on a perfectlyoriented <100> substrate) for thin films. At a thickness of d_(c) ≅45-54Å (4.5-5.4 nm) the easy magnetization direction switched discontinuouslyfrom the 1-10! to the 110! direction and stayed along this direction forfilms as thick as 900 Å (90 nm). The important parameter for a workingsensor device is the coercive field, reported in FIG. 13. The essentialfact is that the coercivity for films with d>d_(c) is high, of the orderof 50 to 100 Oe (4-8 kA/m), and hence these films keep theirmagnetization aligned along the 110! direction even in the externalfields to be sensed. Typical hysteresis loops as observed by themagnetooptic Kerr effect are shown in FIG. 14. Along the 110! directionthe thick film shows a rectangular loop with remanence equal to thesaturation characteristic of an easy direction (FIG. 14A), whereas alongthe 1-10! direction the loop indicates the existence of an intermediateaxis (FIG. 14B). For comparison, the hysteresis along the 1-10!direction for a thin film d=9 Å<d_(c)) (=0.9 nm<d_(c)) shows theswitched easy axis (FIG. 14C). The hysteresis loops thus contain theinformation necessary to determine easy axis and coercivity of aparticular film. Applying this method to the films of variousthicknesses provides a means to determine the values of the criticalthicknesses for a particular film and a particular substrate.

The substrate miscut of 1.6° results in a mean step length ofapproximately 64 Å (6.4 nm) and a mean step length to step height ratioof 35. The observation that a switching of the magnetization directionat large thickness occurs must be attributed to a large surfaceanisotropy. The observation that films as thick as 900 Å (90 nm) showuniaxial behaviour proves that bulk anisotropies are responsible for the110! easy direction in the thick film. Both these anisotropies cannot beidentified with the anisotropies observed on <100> surfaces simplyprojected onto the miscut surface, since the miscut angle is much toosmall to explain the dramatic anisotropy change from thefourfold-symmetric <100> surface to the twofold symmetry on the miscut<100> surface.

A working device consists of a thick pinned Co layer, a nonmagneticspacer, and a free sensing layer. As a first step a thin Co layer asdescribed above was taken as the free layer, and a Cu layer as thespacer. In this configuration it was verified that the observations onthe single Co film are reproduced; i.e. the magnetic properties of a Colayer are not changed by growing it as part of a layeredCo/Cu/Co/Cu<100> sample. FIG. 15A shows a magnetization image of asample with Co14/Cu180/Co90/Cu<100> (non-bracketed numbers indicatethickness in Angstroms, respectively 1.4 nm, 18 nm and 9 nm) exposingthe free layer on the upper part of the image, and for comparison thepinned layer on the lower part. The magnetization directions of the freeand pinned Co layers in this Co/Cu/Co/Cu<100> structure are the same asfor Co layers of the same thicknesses mounted singly on a Cu substrateand circumscribe an angle of 90° . Application of a magnetic fieldlarger than the coercive field of the free layer switches the freelayer, with no influence on the pinned layer, see FIG. 15B. Both layersremain in a single domain state. The thickness values given above areonly for a Co film as the pinned layer.

EXAMPLE III

A similar sample to that in example II was made with the substrate'ssurface deliberately cut at an angle of 1.9° to the nearest plane ofatoms. A ferromagnetic layer mounted on this substrate was found to haveits direction of magnetization pinned. The mean length of the steps onthe substrate's surface corresponding to a miscut of 1.90 isapproximately 54 Å (5.4 nm). The corresponding mean ratio of step lengthto step height is approximately 30.

EXAMPLE IV

Another similar sample to example II was made with the substrate'ssurface cut at an angle of 6.0° to the nearest plane of atoms. Aferromagnetic layer mounted on this substrate was found to have itsdirection of magnetization pinned. The mean length of the steps on thesubstrate's surface corresponding to a miscut of 6.0° is approximately17 Å (1.7 nm). The corresponding mean ratio of step length to stepheight is approximately 10.

EXAMPLE V

In the test sensors described previously, copper was used as thesubstrate. For a magnetoresistive head the metallic single crystallineCu<100> substrate has to be replaced by an insulating or semiconductingmaterial. It has been shown before that Si<100)> serves this purpose. Infact it has even been shown that Co/Cu/Si<100> grows epitaxially. II wasverified that Cu grows in large terraces (several hundred Å (severaltens of nm)) on top of Si<100> in epitaxial <100> orientation, after theCu-silicide formation stops. Therefore the uniaxial behavior of the Cofilms on top of a miscut Cu<100> single crystal is also present if grownon miscut Si<100> wafers covered by a Cu<100> film. The sensor accordingto the invention may therefore have a thin buffer layer, e.g. of copper,immediately above the substrate, onto which the layers of the sensor arethen mounted. Recently it has been shown that thin epitaxial films canbe grown also by sputtering onto oriented substrates. In particular,Co/Cu<100> can be sputtered onto MgO(100). Therefore the invention isalso applicable to sputtered samples grown onto miscut substrates.

The free layer of a GMR head must have a small coercive field, to allowthe rotation of the magnetization direction easily in very small fields.The free Co film of the above structure has not been optimized in thatrespect. In particular the invention is not restricted to Co layers. Thethin Co layer simply shows that a thin magnetic film grown on a miscutsurface is uniaxial. A thin permalloy layer could be selected to do thesame but with smaller anisotropy. The advantage with a permalloy layeris that there the easy magnetization axis can be tailored easilyaccording to the needs by e.g. varying sputtering pressure or applyingmagnetic fields during deposition.

We claim:
 1. A spin valve sensor comprising a substrate (20), the faceof said substrate on which at least two ferromagnetic layers (24, 28)are mounted having a plurality of steps whose mean step height (H) isbetween 1.5 and 30 Å (0.15-3.0 nm) and/or whose mean step length isbetween 10 and 1000 Å (1-100 nm), the thickness of one of the sensor'sferromagnetic layers (24,28) being such that said one layer'smagnetization is pinned.
 2. A spin valve sensor comprising a substrate(20), the face of said substrate on which at least two ferromagneticlayers (24, 28) are mounted having a plurality of steps wherein the meanratio of the step length (L) to the step height (H) of said steps isbetween 5 and 570 and the thickness of one of the sensor's ferromagneticlayers (24, 28) being such that said one layer's magnetization ispinned.
 3. The sensor of claim 1, wherein the substrate is a crystallinesubstrate, wherein the face of the substrate on which at least twoferromagnetic layers are mounted in not coplanar with any of the maincrystallographic planes of the substrate, the thickness of one of saidferromagnetic layers being such that said one layer's magnetizationdirection is pinned.
 4. The sensor of claim 3, wherein the face of acrystalline substrate on which the ferromagnetic layers are mounted liesin a plane, the normal to which plane makes an angle (α) of at least0.1°, preferably at least 0.5°, to the nearest crystal axis of saidcrystalline substrate.
 5. The sensor of claim 1, wherein the thicknessof the first ferromagnetic layer of the spin-valve sensor is greaterthan or equal to the minimum thickness required for said first layer tohave its direction of magnetization pinned, and the thickness of thesecond ferromagnetic layer is less than the minimum thickness requiredfor said second layer to have its direction of magnetization pinned. 6.The sensor of claim 1, wherein the thickness of the second ferromagneticlayer of the spin-valve sensor is greater than or equal to the minimumthickness required for said second layer to have its direction ofmagnetization pinned, and the thickness of the first ferromagnetic layeris less than the minimum thickness required for said first layer to haveits direction of magnetization pinned.
 7. The sensor of claim 1, whereinthe thickness of the first ferromagnetic layer of the spin-valve sensoris greater than the maximum thickness for which said first layer'sdirection of magnetization is pinned, said first ferromagnetic layerthus forming a free layer, and the thickness of the second ferromagneticlayer is less than or equal to the maximum thickness for which saidsecond layer's direction of magnetization is pinned.
 8. The sensor ofclaim 1, wherein the thickness of the second ferromagnetic layer of thespin-valve sensor is greater than the maximum thickness for which saidsecond layer's direction of magnetization is pinned, the secondferromagnetic layer thus forming a free layer, and the thickness of thefirst ferromagnetic layer is less than or equal to the maximum thicknessfor which said first layer's direction of magnetization is pinned. 9.The sensor of claim 1, wherein the thicknesses of the ferromagneticlayers of the spin-valve sensor are such that the direction of the easyaxis of one of said layers makes an angle of about 90° to the directionof the easy axis of the other layer.
 10. The sensor of claim 1, whereinone of the ferromagnetic layers of the sensor is made of a materialwhose magnetic properties are not dependent on said one layer'sthickness.
 11. The sensor of claim 1, wherein the face of the substratecomprises a plurality of flat regions, each flat region consisting of anexposed layer of atoms of said substrate.
 12. The sensor of claim 11,wherein the exposed layer of atoms of the substrate lie in parallelplanes.
 13. The sensor of claim 3, wherein the crystalline substrate ismonocrystalline.
 14. The sensor of claim 1, wherein the steps run inmore than one direction across the surface of the substrate.
 15. Thesensor of claim 1, wherein the ferromagnetic layers of the sensorcomprise or consist of either cobalt, nickel, iron or permalloy.
 16. Thesensor of claim 1, wherein the ferromagnetic layers of the sensorcomprise or consist of cobalt, nickel or iron alloyed with each otherand/or with other non-magnetic materials.
 17. The sensor of claim 15 orclaim 16, wherein the pinned ferromagnetic layer and the freeferromagnetic layer of the sensor comprise or consist of differentmaterials.
 18. The sensor of claim 1, wherein a layer of nonmagneticmaterial between both ferromagnetic layers in the sensor comprises orconsists of copper.
 19. The sensor of claim 1 or claim 4 or claim 13,wherein the substrate comprises or consists of an insulating orsemi-conducting material, preferably silicon.
 20. The sensor of claim 1,wherein the substrate has an additional buffer layer on the surface,onto which buffer layer the first ferromagnetic layer is mounted. 21.The sensor of claim 20, wherein the buffer layer comprises or consistsof copper.